Aftermarket Variable/Multi Speed Blower Kits

Empirical Engineering Answers:

Yes: you will save electricity costs by running your blower on low.

No: you will not save on gas/oil costs by running your blower on low. Because the slower air velocity over the heat element is transferring fewer BTU units into the circulated air and they simply go up the exhaust pipe.

The NET difference of the above two figures is your savings or cost.

I am currently heavily invested in studying and learning the subject as I commit myself to reducing my winter energy costs this year.

With heating oil at $4.50/gal I can assure you without measuring anything that running my blower on low is NOT more efficient from a $$ perspective. I'm in the process of re-wiring my blower to run on high with the furnace. In support of this: I found the mfr owner's manual for my furnace and in it, it states, "if you have an air-conditioning coil installed, you should switch the furnace blower speed to high to accommodate the decreased air-flow caused by the cooling coil in the plenum."

I have also measured that on low speed my furnace requires AT-LEAST 1 hour of constant running to raise my home temp by 6 deg F. On high speed it takes 30-40 min. With a one-gallon-per-hour burner nozzle that saves me AT-LEAST 30% on fuel oil plus another 20-30 min of blower run time or at least 30-50% run time savings. Now, I haven't done the measurement to learn how much electricity my blower uses on low vs high. But I'm reasonably sure it uses less to run on high vs low then it uses to run on low vs OFF. I do know that my entire furnace runs safely on a 15amp breaker with the blower on high, the burner running, the humidifier running, and the water pump to empty the humidifier running. Thus, my blower can't be using that much electricity on high.

So clearly, running your furnace blower on low is only more efficient IF-and-ONLY-IF the cost of the heating fuel is much much less then the cost of the electricity to run the blower that little bit faster.

Does the ratio of static pressures between two stories differ at different blower speeds? That's a very interesting question, but I believe it's a moot point. Any dramatic difference would mean your system isn't balanced correctly in the first place. (needs zoning, split systems, or an actual balancing)

Stack effect will cause plenty of heat to go upstairs, while the cold air will settle. I'd be more concerned that it won't cool right. Except that furnaces usually only run at high speed for AC because cold air is denser, making it harder to push. If your old system heats and cools just fine I don't foresee a problem.

Without being in the correct centralized location (which is somewhere on the first floor of a two story), the thermostat will not control the house properly. In low-fire mode, indeed less air will go upstairs and downstairs, per minute. Blowing softly is what makes it more comfortable. It will run longer and more often, reducing the intermittent temperature differential, keeping the house nice and even.

So no, the same amount of 'less' air will be coming out below as above. Your contractor either is trying to up-sell you into a split system (or knows more than I do) or convince you that zoning is required (a possibility, however this can be retrofitted later at your discretion). Either of these additions would dramatically increase the cost.

Unless your house has automatic zoning and was balanced by a genius, it will always be hotter on the second floor in the winter, and colder on the first in the summer, just like everyone else's.